In many cases antennas are installed to non-rigid or non-stationary structures, the motion of which may result in time-varying antenna orientation as the structure is exposed to various forces. Consequently, the direction of the radiation pattern of such an antenna (for example as measured with respect to the main beam peak, for a directional antenna) will also vary over time. One example of such an installation case is when an antenna is mounted on a mast or tower which moves (sways) when exposed to varying wind conditions (wind load). Typically, this motion results in both a translation and a rotation of the antenna. Maximum antenna sways are on the order of ±1 degree (or smaller) for typical base station antenna installations.
The extent to which the antenna motion influences system performance depends on several things, the most important of which may be the antenna elevation beamwidth, when considering the rotation aspect of motion. When the antenna rotation angle, and the corresponding beam-squint, is significantly less than the elevation beamwidth (in the plane of beam-squint), its effects on system performance can typically be ignored. This is the case for almost all antenna installations used in existing base stations for cellular communications systems. However, in order to improve coverage, one increasingly popular solution is to use antennas with higher gain compared to typical gain figures of conventional basestation antennas. These new higher gain antennas are often realized with very narrow elevation half-power beamwidths. An example of such antennas is disclosed in the published international patent application WO 2006/065172 (reference [1]), assigned to Telefonaktiebolaget LM Ericsson.
Narrow elevation beamwidths accentuate the effects of antenna (mounting structure) motion and may cause problems if not carefully dealt with. There are already existing antenna installations in which mast motion is taken into account when choosing installation height. One example is radio link antennas, which in some cases are intentionally installed at positions on the mast or tower where rotation is low, e.g. midway between the mounting structure resonance nodes, in order to ensure link transmission quality.
The translation aspect of antenna structure motion can typically be ignored, since the translation is relatively low-speed and therefore produces negligible translation-dependent effects (for example Doppler shift).
For a given mast or tower structure, it may not be possible (or suitable) to use an antenna with a desired, narrow, elevation beamwidth, because of the risk of motion-related performance degradation.
The complementary problem description is that the installation and use of a desired narrow-beam antenna may require a more rigid (expensive) mast or tower structure, or an antenna installation height that is suboptimal under ideal conditions but necessary to ensure desired performance under non-ideal conditions.
A solution to this problem is disclosed in U.S. Pat. No. 5,894,291 (reference [2]), which shows a method for dynamically counteracting antenna tower sway by modifying an antenna drive signal so as to electrically steer an active antenna mounted on said tower towards a desired direction. Furthermore, it discloses one or more motion sensors configured to detect antenna tower motion, as illustrated in FIG. 1.
A problem with the prior art [2] is that the method only compensate for antenna structure motion, i.e. the antenna tower. The motion of the antenna may differ from motion in the antenna structure since the sway/tilt of antenna may not be deterministically dependent on the sway/tilt of the mounting structure (tower/mast), particularly not for installation on different types of structures. Compensation by redirecting the beam is only achieved for a rotational movement in the direction of the beam, as illustrated by FIGS. 2a and 2b. 
Thus, there is a need to provided a more sophisticated method for compensate antenna motion.